September 2010 - Haskell-Cafe

Disable LINE Pragma handling in GHC
by JP Moresmau 13 Sep '10

13 Sep '10

Hello fellow Haskellers, In EclipseFP we use the GHC API for IDE related stuff like syntax highlighting and code outlines. However, I ran into something funny yesterday: when a source file contains LINE pragmas ( http://www.haskell.org/ghc/docs/6.12.2/html/users_guide/pragmas.html#line-p…) all the locations for tokens are changed to reflect the pragmas information. While this is great in the "normal' GHC usage, this is not so great for us, because we're interested in that source code, not in original code. I haven't seen any flag to turn that behavior off in the docs, nor in the Lexer code, but have I missed something? Can I tell GHC to just ignore these pragmas? I suppose even using GHC for building and something else (haskell-src-exts?) for code handling would leave us with compilation messages at the wrong place. Thanks, -- JP Moresmau http://jpmoresmau.blogspot.com/

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Subcategories in Hackage
by Daniel Díaz 12 Sep '10

12 Sep '10

Hi, all! Now, with the approaching new Hackage server, I thought that to organize packages into subcategories would be useful. Categories like "System" or "Text", with almost 200 packages, make more difficult to find what you want. "Data" category has currently 326 packages, many of them with very different roles. Perhaps, this is madness, but I wanted to read other opinions on this topic. -- Daniel Díaz

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darcs in 2010: talk/video online
by Eric Kow 12 Sep '10

12 Sep '10

Dear Haskellers, Derek asked me if I could give a 5 minute talk at AngloHaskell last Friday about the Darcs project. It took an hour and a half. I've redone the talk on YouTube, basically just stepping through the slides and talking over them. It's about an hour long and you can watch it here: http://www.youtube.com/watch?v=iUytayyTGTU You may also prefer to just zoom through the slides yourself (about 10 minutes): http://prezi.com/is5nzudw8jxq/darcs-in-2010/ Many thanks to Derek for organising the AngloHaskell event and for thinking hard about how to make it work better! Thanks also to the Darcs hackers who did the actual work behind this talk, and to the user community for all your support and feedback. Eric PS: the talk segments are: 1. http://www.youtube.com/watch?v=iUytayyTGTU 2. http://www.youtube.com/watch?v=ZXjTmWD81JA 3. http://www.youtube.com/watch?v=1lXOD3fLwU8 4. http://www.youtube.com/watch?v=aueSoZShobo 5. http://www.youtube.com/watch?v=-JxJKMSRmBY 6. http://www.youtube.com/watch?v=zdImEqRmw7I -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> For a faster response, try +44 (0)1273 64 2905 or xmpp:kowey@jabber.fr (Jabber or Google Talk only)

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ANNOUNCE: GotoT-transformers version 1.0
by Gregory Crosswhite 12 Sep '10

12 Sep '10

People want to believe that Haskell is a better language than C, but how could this possibly be true when Haskell lacks the very basic "goto" feature??? If the world is going to take Haskell seriously, then this serious blight needs to be addressed immediately! Thus I proud to present to you the "GotoT-transformers" package which provides this missing functionality and so finally makes Haskell a serious contender with C. On a more serious note, there are times when it can be useful to not only abort a computation in the middle, but to then transfer the flow of execution to another computation: do [...stuff...] when condition1 $ goto x [...more stuff...] when condition2 $ goto y [...more stuff...] when condition3 $ goto z Towards this end, I developed this package which provides a GotoT monad transformer and a "goto" function that takes a monadic computation and returns a monadic value that has the effect of transferring control to the new computation. Obviously this functionality can be abused, but when it is the natural fit I believe that it is much better to use it directly then to go through all sorts of contortions resulting in uglier code just to avoid it on principle. The best implementation I could think of was to use a "trampoline" approach. The GotoT monadic transformer is represented by a type which contains (wrapped inside the inner monad) either a pure value or a monadic computation. The runGotoT function operates by returning the value if it is pure, and evaluating the monadic computation and feeding the result back into itself otherwise. The "trampoline" term refers to the fact that the flow of execution bounces back to the runGotoT function as long as the computation keeps calling the "goto" function. I hope that the community not only finds this functionality useful, but also that does not kill me for committing the egregious act of bringing "goto" into the Haskell world. Cheers, Greg

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Cost: (:) vs head
by michael rice 12 Sep '10

12 Sep '10

Which of these would be more costly for a long list? f :: [Int] -> [Int] f [x] = [x] f (x:xs) = x + (head xs) : f xs f :: [Int] -> [Int] f [x] = [x] f (x:y:xs) = x + y : f (y:xs) Michael

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Small question on concurrency
by Arnaud Bailly 12 Sep '10

12 Sep '10

Hello Haskellers, Having been pretty much impressed by Don Stewart's "Practical Haskell" (http://donsbot.wordpress.com/2010/08/17/practical-haskell/) I started to write a Haskell script to run maven jobs (yes, I know...). In the course of undertaking this fantastic endeavour, I started to use the System.Process.readProcessWithExitCode function, but following the advice in the comment for this function, I rolled my own stuff and ended up writing the following: > doRunMvnInIO pom args filters e > = do (Just inh, Just outh, Just errh, pid) <- > createProcess (proc (maven e) (["-f", pom] ++ args)) { std_in = CreatePipe, > std_out = CreatePipe, > std_err = CreatePipe } > waitQ <- newEmptyMVar > > mapM (printAndWait waitQ) [outh, errh] > > hClose inh > -- wait on the process > waitForProcess pid > where > printAndWait waitQ hdl = do out <- hGetContents hdl > forkIO (mapM (putStrLn) (filter filters $ lines out) >> putMVar waitQ ()) > takeMVar waitQ > hClose hdl This is actually a rewrite of the following function: > doRunMvnInIO' pom args filters e > = do (Just inh, Just outh, Just errh, pid) <- > createProcess (proc (maven e) (["-f", pom] ++ args)) { std_in = CreatePipe, > std_out = CreatePipe, > std_err = CreatePipe } > waitQ <- newEmptyMVar > > mapM (printAndWait waitQ) [outh, errh] >>= mapM (\_ -> takeMVar waitQ) > > hClose inh > hClose outh > hClose errh > -- wait on the process > waitForProcess pid > where > printAndWait waitQ hdl = do out <- hGetContents hdl > forkIO (mapM (putStrLn) (filter filters $ lines out) >> putMVar waitQ ()) What surprised me is that I would expect the behaviour of the two functions to be different: - in doRunMvnInIO, I would expect stdout's content to be printed before stderr, ie. the 2 threads are ordered because I call takeMVar in between calls to forkIO - in doRunMvnInIO', this is not true and both theads run concurrently. but actually there does not seem to be a difference: printing is still interleaved in both functions, AFAICT. I would welcome any help on this. Best regards, Arnaud

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Re: Haskell web developer position at CREATE-NET
by Chris Done 11 Sep '10

11 Sep '10

Here at CREATE-NET, a research center located in Trento, Italy, we have a position open for a web developer. The following are essential: 1. Experience writing standard-compliant, semantic and accessible web documents with (X)HTML/CSS2. 2. Strong JavaScript experience. 3. Experience with cross-browser (IE7-8/Firefox/Chrome/Safari/Opera) development, and dealing with or avoiding the inevitable IE quirks -- we do not maintain for IE6. 4. Strong Haskell and/or Common Lisp development experience (preferably both). 5. English language fluency (spoken and written) is essential, as are good writing and oral communication skill. 6. Qualified to work in Europe. PostgreSQL experience advantageous. Must be willing to re-locate in order to work in the Trento CREATE-NET buildings. We offer competitive compensation and a stimulating work environment in the research and innovation sector. Please provide examples of your web development work and Haskell/Common Lisp projects with a brief explanation for each or link for it. Submit your application to: itjobs(a)create-net.org Skiing and snowboarding is popular here in the winter. ;-) Cheers, Chris Done CREATE-NET P.S. Note that although we are a research center, this opening is for a developer position, not research. You are welcome to write papers but this cannot be your main activity.

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Trying to compile Glade Gtk2Hs demo / cabal install glade problem
by Peter Schmitz 11 Sep '10

11 Sep '10

(I am posting this to Cafe and Gtk2Hs Users; if you subscribe to both, please reply to Cafe; but I will see your replies either place; thanks much.) I am trying to compile the Gtk2Hs demo program GladeTest.hs (located in the ...\demo\glade dir), under MS Windows XP. The error I get is: H:\proc\tools\Gtk2Hs\demo\glade>ghc --make GladeTest.hs GladeTest.hs:4:7: Could not find module `Graphics.UI.Gtk.Glade': (--Note:-- I am using H: instead of C: for reasons described later.) I did these okay: cabal install cabal-install cabal update cabal install gtk2hs-buildtools cabal install gtk and I can compile and run some other Gtk2Hs demos (that do not use Glade) okay. In general, I am assuming that I need to "cabal install foo" when Haskell module foo cannot be found during compile. (Please correct me if I am wrong.) I get an error during the glade install: H:\proc\dev\cmd>cabal install glade ... Configuring glade-0.11.1... setup.exe: The pkg-config package libglade-2.0 version >=2.0.0 is required but it could not be found. ... (the full output is listed below) I think I have package libglade-2.0 installed, because I installed the GTK+ and Glade bundle from: http://ftp.gnome.org/pub/GNOME/binaries/win32/glade3/3.6/glade3-3.6.7-with-… at: http://ftp.gnome.org/pub/GNOME/binaries/win32/glade3/3.6/ at: http://glade.gnome.org/ That install created tree: H:\proc\tools\Gtk+\ which includes these: H:\proc\tools\Gtk+\include\libglade-2.0\ and H:\proc\tools\Gtk+\glade-3.exe H:\proc\tools\Gtk+\bin\libglade-2.0-0.dll H:\proc\tools\Gtk+\lib\libglade-2.0.a H:\proc\tools\Gtk+\lib\pkgconfig\libglade-2.0.pc So, I'm thinking maybe my install of Gtk+ (just running glade3-3.6.7-with-GTK+.exe) was lacking somehow, or cabal somehow doesn't see my Gtk+ dir (during "cabal install glade") ? So, I tried editing my cabal config so it would see my Gtk+ tree. I uncommented and edited these lines in the main, un-indented section: extra-include-dirs: H:\proc\tools\Gtk+\include extra-lib-dirs: H:\proc\tools\Gtk+\lib but it doesn't seem to help. (my full cabal config listing is below) Regarding my use of H: instead of C:, I have a non-standard configuration of the Haskell Platform and cabal. I need to develop (Haskell code that uses Gtk2Hs, Glade, etc.) in a MS Windows network, from any of several PCs (one at a time), each of which uses a common tools tree. So, instead of installing the Haskell Platform, cabal, Gtk+, etc. to C:, I have installed them to a network drive (H:). So, I have: H:\proc\tools\Haskell Platform H:\proc\tools\Gtk+ H:\proc\tools\cabal etc. Any help or pointers would be appreciated. Thanks very much, -- Peter --- Following is various config and output info --- My PATH is: ++++++++++++++ SET P_TOOLS=H:\proc\tools SET HASK_PLAT=%P_TOOLS%\Haskell Platform\2010.1.0.0 PATH %HASK_PLAT%\bin; %P_TOOLS%\cabal\bin; %P_TOOLS%\gtk\bin; %HASK_PLAT%\lib\extralibs\bin; %HASK_PLAT%\mingw\bin ++++++++++++++ Some of my environment vars are: ============== REM This is so Cabal will get its config from the Haskell Platform, REM rather than C:\Documents and Settings\pschmitz\Application Data\cabal SET CABAL_CONFIG=%P_TOOLS%\cabal\config CABAL_CONFIG=H:\proc\tools\cabal\config PKG_CONFIG_PATH=H:\proc\tools\Gtk+\lib\pkgconfig GTK_HOME=\\fbpxfile01\users$\pschmitz\proc\tools\Gtk+ HASK_PLAT=H:\proc\tools\Haskell Platform\2010.1.0.0 INCLUDE=\\fbpxfile01\users$\pschmitz\proc\tools\Gtk+\include LIB=\\fbpxfile01\users$\pschmitz\proc\tools\Gtk+\lib Path=H:\proc\tools\Haskell Platform\2010.1.0.0\bin;H:\proc\tools\cabal\bin;H:\proc\tools\Gtk+\bin; H:\proc\tools\Haskell Platform\2010.1.0.0\lib\extralibs\bin;H:\proc\tools\Haskell Platform\2010.1. 0.0\mingw\bin; ============== The full output from "cabal install glade" is: ----------------------------- H:\proc\dev\cmd>cabal --version cabal-install version 0.8.2 using version 1.8.0.2 of the Cabal library H:\proc\dev\cmd>cabal list glade * glade Synopsis: Binding to the glade library. Latest version available: 0.11.1 Latest version installed: [ Not installed ] Homepage: http://www.haskell.org/gtk2hs/ License: LGPL-2.1 ... H:\proc\dev\cmd>cabal info glade * glade (library) Synopsis: Binding to the glade library. Latest version available: 0.11.1 Latest version installed: [ Not installed ] Homepage: http://www.haskell.org/gtk2hs/ Bug reports: http://hackage.haskell.org/trac/gtk2hs/ Description: This library allows to load externally stored user interfaces into programs. This allows alteration of the interface without recompilation of the program. Category: Graphics License: LGPL-2.1 Author: Manuel M T Chakravarty Maintainer: gtk2hs-users(a)sourceforge.net Source repo: http://code.haskell.org/glade/ Dependencies: base >=4 && <5, array -any, containers -any, haskell98 -any, mtl -any, glib >=0.11 && <0.12, pango >=0.11 && <0.12, cairo >=0.11 && <0.12, gtk >=0.11 && <0.12 Cached: Yes Modules: Graphics.UI.Gtk.Glade H:\proc\dev\cmd>cabal install glade Resolving dependencies... C:\DOCUME~1\pschmitz\LOCALS~1\Temp\glade-0.11.13100\glade-0.11.1\Gtk2HsSetup.hs:25: warning: #warning Setup.hs is guessing the version of Cabal. If compilation of Setup.hs fails use -DCABAL_VERSION_MINOR=x for Cabal version 1.x.0 when building (prefixed by --ghc-option= when using the 'cabal' command) [1 of 2] Compiling Gtk2HsSetup ( C:\DOCUME~1\pschmitz\LOCALS~1\Temp\glade-0.11.13100\glade-0.11.1\Gtk2HsSetup.hs, C:\DOCUME~1\pschmitz\LOCALS~1\Temp\glade-0.11.13100\glade-0.11.1\dist\setup\Gtk2HsSetup.o ) [2 of 2] Compiling Main ( C:\DOCUME~1\pschmitz\LOCALS~1\Temp\glade-0.11.13100\glade-0.11.1\Setup.hs, C:\DOCUME~1\pschmitz\LOCALS~1\Temp\glade-0.11.13100\glade-0.11.1\dist\setup\Main.o ) Linking C:\DOCUME~1\pschmitz\LOCALS~1\Temp\glade-0.11.13100\glade-0.11.1\dist\setup\setup.exe ... Configuring glade-0.11.1... setup.exe: The pkg-config package libglade-2.0 version >=2.0.0 is required but it could not be found. cabal: Error: some packages failed to install: glade-0.11.1 failed during the configure step. The exception was: ExitFailure 1 ----------------------------- My complete Cabal config follows. The email may alter the indentation, but I was careful to maintain the original indentation when editing. -- ---------------------------------------------------------------------------------------------- -- This is the configuration file for the 'cabal' command line tool. -- The available configuration options are listed below. -- Some of them have default values listed. -- Lines (like this one) beginning with '--' are comments. -- Be careful with spaces and indentation because they are -- used to indicate layout for nested sections. remote-repo: hackage.haskell.org:http://hackage.haskell.org/packages/archive remote-repo-cache: H:\proc\tools\cabal\packages -- local-repo: -- verbose: 1 -- compiler: ghc -- with-compiler: -- with-hc-pkg: -- scratchdir: -- program-prefix: -- program-suffix: -- library-vanilla: True -- library-profiling: False -- shared: False -- executable-profiling: False -- optimization: True -- library-for-ghci: True -- split-objs: False -- executable-stripping: True -- user-install: True -- package-db: -- flags: -- -- -------------------------------- -- extra-include-dirs: -- extra-lib-dirs: extra-include-dirs: H:\proc\tools\Gtk+\include extra-lib-dirs: H:\proc\tools\Gtk+\lib -- -------------------------------- -- -- constraint: -- cabal-lib-version: -- preference: -- documentation: False -- doc-index-file: $datadir\doc\index.html -- root-cmd: -- symlink-bindir: build-summary: H:\proc\tools\cabal\logs\build.log -- build-log: remote-build-reporting: anonymous -- username: -- password: install-dirs user prefix: "H:\\proc\\tools\\cabal" -- bindir: $prefix\bin -- libdir: $prefix -- libsubdir: $pkgid\$compiler -- libexecdir: $prefix\$pkgid datadir: "H:\\proc\\tools\\Haskell Platform\\2010.1.0.0" -- datasubdir: $pkgid -- docdir: $prefix\doc\$pkgid -- htmldir: $docdir\html -- haddockdir: $htmldir install-dirs global prefix: "H:\\proc\\tools\\Haskell Platform\\2010.1.0.0" -- bindir: $prefix\bin -- libdir: $prefix -- libsubdir: $pkgid\$compiler -- libexecdir: $prefix\$pkgid datadir: "H:\\proc\\tools\\Haskell Platform\\2010.1.0.0" -- datasubdir: $pkgid -- docdir: $prefix\doc\$pkgid -- htmldir: $docdir\html -- haddockdir: $htmldir -- ---------------------------------------------------------------------------------------------- -- end of post: Trying to compile Glade Gtk2Hs demo / cabal install glade problem --

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Fwd: Announcement: Pearls of Functional Algorithm Design
by Don Stewart 11 Sep '10

11 Sep '10

----- Forwarded message from Richard <richard(a)normab.freeserve.co.uk> ----- To: haskell(a)haskell.org Subject: [Haskell] Announcement My book "Pearls of Functional Algorithm Design" will be published by CUP this month. See http://www.comlab.ox.ac.uk/news/237-full.html Richard Bird ----- End forwarded message ----- For those not familiar with the functional pearl series, I list some of Richard Bird's here. http://www.reddit.com/r/programming/comments/dcimd/prof_richard_birds_pearl… The essence of elegant functional programming. -- Don

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Relative files paths
by Andrew Coppin 11 Sep '10

11 Sep '10

Suppose that I'm in foo/bar/box1/top and I want to access a file in foo/bar/box2/side. How do I construct the shortest possible relative path from one to the other? The filepath package doesn't seem to provide a function to do this. (The correct answer is obviously "../../box2/side" in this instance, but how do I compute that in general?)

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